1. FIELD OF THE INVENTION
This invention relates to a method of repairing a mask and, more particularly, to a method of repairing a shifter pattern defect or more specifically a clear defect of a phase shifting mask used in a photolithography process for manufacture of semiconductor elements.
2. DESCRIPTION OF RELATED ART
The resolution limit R (.mu.m) of a lithography technique using an optical contraction type stepper is expressed as EQU R=k.sub.1 .lambda./NA, k.sub.1 =0.5
where .lambda. represents wavelength (.mu.m) and NA represents the numerical aperture of the lens. In accordance with this equation, resolution limit of the optical lithography is minimized by reducing the exposure wavelength, increasing the value of NA and/or reducing the constant k.sub.1 of the resist dependent upon the process. Presently, a stepper using i-line (.lambda.=0.365 .mu.m) and having NA=0.5 is available and k.sub.1 =0.5 is possible. A resolution of about 0.4 .mu.m is thereby attained. To further reduce the resolution limit, the exposure wavelength may be reduced or the value of NA may be increased. However, technical difficulties in obtaining a suitable light source and a suitable lens and a reduction in the depth of focus .delta.=.lambda./2NA.sup.2 are thereby encountered.
To overcome these problems, a phase shift exposure method has been proposed. Examples of this method are disclosed in Japanese Published Patent Applications 57-62052 and 58-173744.
FIGS. 1A to 1C show the principle of a conventional photomask method and FIGS. 2A to 2C show the principle of the phase shift exposure method. FIGS. 1A and 2A are sectional side views of mask substrates, FIGS. 1B and 2B show the strengths of electric fields on masks, and FIGS. 1C and 2C show the intensity of light on wafers.
First, these methods will be described below with respect to transfer of a pattern finer than the optical lithography resolution limit. In the case of the photomask method, the electric fields of the light passing between mask patterns (main patterns) 2 of a mask formed on a mask substrate 1 (formed of, e.g., SiO.sub.2) are spatially separated from each other as indicated by FIG. 1B. However, the light intensity (FIG. 1C) of the light transmitted through the optical system onto the wafer (not shown) is continuously distributed, and it is not possible to form the pattern image.
On the other hand, in the phase shift method, pattern transfer is effected by using a mask having phase shifting patterns (shifter) 3 which are SiO.sub.2 films or the like formed at alternate pattern spaces 2a of a mask pattern 2, as shown in FIG. 2A. The shifter patterns 3 shift the phase of light through 180.degree.. The electric fields of the light passing through this mask are as shown in FIG. 2B, with their phases alternately inverted. If the pattern images of this mask are projected by using the same light source as the photomask method, the amounts of light respectively forming the image patterns are cancelled out at the positions where the image patterns overlap, because the phases of the overlapping light are opposite to each other. A split intensity pattern such as that shown in FIG. 2C is thereby obtained. For this reason, the phase shift exposure method ensures a higher resolution in comparison with the photomask method, as confirmed by experiment, and reduces the minimum pattern resolution width by substantially by half.
Next, a method of repairing pattern defects of the phase shifting mask used in accordance with the phase shift exposure method will be described below. FIGS. 3A to 3C show the principle of a method of repairing an opaque defect of phase shifter patterns, and FIGS. 4A to 4C show the principle of a method of repairing a transparent defect of phase shifter patterns. Details of a mask substrate 1 and other members shown in these figures are as described below. Mask patterns 2 formed of Cr or MoSi are provided on the mask substrate 1. Phase shifter patterns 3 formed of SiOx or PMMA (polymethylmethacrylate) are formed at predetermined positions between the mask patterns 2. A reference numeral 4 in FIG. 3A designates an opaque defect region of the phase shifter pattern 3. The opaque defect region 4 is formed of SiOx or PMMA like the phase shifter pattern 3.
This opaque defect is repaired as described below. As shown in FIG. 3B, the opaque defect region 4 is irradiated and scanned with a focused ion beam (FIB) 5, etched (milled), and removed. For example, a 1 .mu.m.sup.2 defect can be removed in several minutes by using a 30 keV Ga.sup.+ ion beam (beam current: 300 pA). PMMA is regarded as more suitable for forming the phase shifter patterns 3 in consideration the detection of etching end points. This is because secondary ions are used to detect the end points and because the change in the yield of secondary ions at each end point is small if a material such as SiOx similar to the material of the mask substrate 1 is used. FIG. 3C shows the result of repair of the opaque defect. Residues 4a of the opaque defect are left on the mask patterns 2, which is unimportant since the mask patterns 2 are not optically transmissive.
A method of repairing a transparent defect will be described below with reference to FIGS. 4A to 4C. A reference numeral 6 in FIG. 4A designates a clear defect region to be repaired. To repair this clear defect, the transparent defect region 6 is irradiated with an ion beam 5 in a reactive gas atmosphere to deposit a shifter material 7 on the transparent defect region 6, as shown in FIG. 4B. This method is a film deposition method using the ion beam assist method. The same ion beam 5 used for the opaque repair is used to scan the transparent defect region 6 a plurality of times. During this scanning, a reactive gas is led through a nozzle 8 to a position in the vicinity of the transparent defect repair region and is blown out as a nozzle beam 9. The distance between the nozzle 8 and the mask substrate 1 is about several hundred microns, and the nozzle 8 is positioned so that the defect region is located just in front of the nozzle 8 in the axial direction of the same. SiH.sub.4 and O.sub.2 may be used for the reactive gas. These gases decompose and react with each other, and SiO.sub.2 (or SiOx) is thereby deposited as the shifter material 7 on the clear defect region 6. FIG. 4C shows the result of repair of the transparent defect. Since SiOx is deposited in this case, the thickness of the shifter material layer is about 100 nm. Because the composition and the structure of deposited SiOx change according to the conditions of the irradiation of the ion beam 5, the reaction gas atmosphere and so on, it is necessary to control and optimize the thickness of the deposited SiOx.
The above-described mask repair method is disadvantageous as described below. A toxic and pyrophoric gas such as SiH.sub.4 is used to repair a transparent defect of phase shifter patterns. It is therefore necessary to provide a complicated gas supply apparatus to ensure security. This apparatus needs to be controlled carefully.
The film deposition method using the ion beam assist method is inferior in accuracy in comparison with the etching. There is therefore a problem that the accuracy of transparent defect repairs of phase shifter patterns are lower than the accuracy of opaque defect repairs.